1. Field of Invention
The invention relates to interactive elements commonly used in recording data on storage media in information storage devices. Specifically, the invention relates to a head design for use with recording an information apparatus utilizing a liquid bearing between the interactive element head and the storage disks.
2. Description of the Related Art
Designers of information storage technology constantly seek to improve data storage capacity and performance of information storage devices. In one aspect, this involves improving the relationship between an interactive element and the storage medium.
In Winchester-type magnetic storage devices, the interactive element is a read/write head which interacts with a magnetic storage disk. The read/write head is generally comprised of an active element, such as a transducer, mounted on a slider. Although continual contact between the interactive element and the magnetic storage medium is advantageous from a magnetic perspective, such arrangement is undesirable because wear and material interactions lead to degraded system reliability and performance. Accordingly, one paramount consideration in designing magnetic recording systems is the maintenance of spacing between the head and medium. In conventional drives, the read/write head "flies" over the disk-shaped recording medium by compressing a fine layer of air to form a very low friction gas bearing that maintains the head above, and out of contact with, the recording medium. The head-disk interface distance or "flying height" is determined by such factors as the shape, load and size of the head.
The read/write head itself generally includes a transducer element to read and write flux transitions into the magnetic material of the recording medium. Data is stored on the medium by the fringing magnetic field of the transducer element, generally a coil surrounding an active core providing two magnetic poles separated by a non-magnetic gap which is preferably oriented perpendicular to the disk surface. It is desireable to reduce the flying height of the recording head to increase the field gradient of the fringing flux of the transducer element incident to the storage medium and to increase the flux from the medium which reaches the head during playback.
FIGS. 1 and 2 illustrate, in general form, the interaction between a conventional air bearing read/write head and a magnetic storage disk, and a conventional air bearing read/write head design, respectively. The interface between a rigid disk medium 10 and a transducer 21, mounted on a slider 22, is maintained in an assembly including a means for rotating the disk 10, the means including a spindle 12 which fits through a central circular hole in the middle of the disk 10, and a spindle rotator 14. Slider 22 is normally loaded by application of a force applied to slider 22 and directed toward disk 10. The force is applied by an assembly including actuator 16 swing arm 18, and gimballed spring suspension mechanism 20. The actuator pivots swing arm 18 to selectively position transducer 21 radially with respect to disk 10. Gimballed spring suspension mechanism 20 loads slider 22 against disk 10 when disk 10 is stationary. When disk 10 rotates, an air bearing is generated which counterloads slider 22, lifting and maintaining slider 22 and transducer 21 above the surface of disk 10 to a flying height. The spacing between transducer 21 and the surface of disk 10 is generally determined by the amount of loading exerted on slider 22, by the spring suspension, the aerodynamic design of slider 22 and the rotational velocity of the disk. It is to be understood that FIG. 1 is representative of the minimum complement of parts used to establish the operational interface between transducer 21 and disk 10. In fact, a headdisk assembly normally makes provision for a plurality of parallel-spaced, coaxial disks which rotate on a common spindle together with a ganged array of transducer heads controlled by one or more actuator assemblies.
While the disk rotates, conventional means, not shown, are used to operate the transducer 21 to electromagnetically read signals from, or write signals to, the disk. For this purpose, the transducer includes a wound core with a recording gap. Each head is thus comprised of two major parts: the slider, which provides the air bearing surface; and the transducer element, usually comprising a coil winding and recording gap, with the coil coupled to the control electronics of the disk drive with recording of data generated by the flux changes inducing magnetic changes in the recording medium. Rails, ramps and/or pads are incorporated onto the bottom portion of the slider to optimize flight characteristics for the head. The transducer is generally positioned at the trailing edge of the slider relative to the motion of the slider with respect to the disk.
FIG. 2 illustrates a typical prior art read/write head support structure. Slider 22 is suspended on a gimbal 24, which allows slider 22 to be parallel to disk 10 in response to forces applied to the slider in directions normal to disk 10, while preventing lateral motion and yaw of the slider.
Slider 22 includes a pair of elongated rails 23, each with a flat bearing surface. A narrow middle rail 25 is provided, also with a flat surface. The leading edges of side rails 23 are beveled at 27, as is the leading edge of the center rail at 29. Transducer 21 is mounted on slider 22 to place the recording gap 30 near the trailing edge of center rail 25, while transducer windings 32 are positioned below the recording gap on the trailing edge of the slider assembly beneath an anterior extrusion of center rail 25. When disk 10 rotates, a layer of air (the "air bearing") builds up at the interface between the slider of FIG. 2 and disk 10. The built-up layer of air reverse loads the slider on the flat bearing surfaces of rails 23, lifting the slider off of, or away from disk 10. When the disk achieves its operational rotational velocity, the slider of FIG. 2 is borne on the counter-loading layer of air, which now functions as a bearing in supporting the slider during relative motion between it and disk 10. When rotation of the disk ceases, the flow of air between the slider and the disk ceases removing the counter-loading force on the slider and permitting it to contact the surface of disk 10.
In addition to the three-rail slider shown in FIG. 2 the art also encompasses other slider configurations, some of which eliminate the center rail and place the electromagnetic transducer element in one of the two laterally spaced slider rails 23.
An alternative air bearing design utilizing "feet" or pads is shown in U.S. Pat. No. 4,757,402, issued to Mo. In order to reduce the effects of relative air motion on the flying height of the slider, a plurality of head designs having one, two, four or six cylindrically-shaped pads are shown. Notably, this design eliminates a "railed" configuration altogether in favor of the cylindrical pads.
Co-pending U.S. patent application Ser. No. 07/590,608 and U.S. Pat. No. 5,097,368, teach data storage devices utilizing a liquid bearing lubricant at the head-disk interface to reliably achieve a lower flying height than possible with an air bearing interface. In particular, a liquid bearing interface, having non-Newtonian characteristics with respect to the relationship between the shear stress and shear rate is utilized in a disk drive storage device to achieve flying heights on the order of 1.mu." or less. Under conditions encountered in operation in a liquid disk drive, the liquid is subjected to a shear rate which is high enough to cause its viscosity to depend on shear rate. Viscosity is defined as the proportionality constant between shear stress (F/A) and the velocity gradient. This relationship is symbolized by equation (1), wherein: EQU F/A=u dv/dy (1)
where F is the force exerted on a stationary plate having a face with an area A by a parallel plate moving at a velocity v and spaced from the face at a distance (i.e., head-to-disk spacing) y by a liquid of viscosity u. For small y, the shear rate dv/dy is linear with y, and is v/y. When u is a constant, the liquid is said to be Newtonian, and the classical equation of hydrodynamics, the Navier-Stokes equation, is valid. The Navier-Stokes equation is not valid for any fluid that departs from constant u; in particular, the equation is not valid for fluids that depart so far from the constant as to be called "pseudo-plastic" or "plastic." FIG. 3 illustrates the relationship of Newtonian and pseudo-plastic and plastic liquids.
For definition and explanation of non-Newtonian fluid flow, reference is made to the Chemical Engineer's Handbook, Fifth Edition, Robert H. Perry, et al., Editors, 1973, at pp. 5-38 through 5-40.
FIG. 3 illustrates the shear stress of a liquid as a function of time rate of deformation of the liquid (shear rate). In FIG. 3, the shape of each curve directly represents the change in viscosity of the liquid. The curve labeled "Newtonian" shows a constant viscosity of a value corresponding to the slope of the curve. The curves labeled "pseudo-plastic" and "plastic" indicate liquids whose viscosity apparently decreases with an increase in shear rate.
The plastic or pseudo-plastic nature of the liquid bearing is important since it allows very small power dissipation in the head-disk interface at the speeds and small spacings necessary for a very high density of information storage on the disk.
In this regard, the power dissipated at the interface between the head and disk is given by equation (2), wherein: EQU P=Fv. (2)
From equations (1) and (2), it is possible to calculate the drag force on a head, and the power dissipated in rotating a disk when the liquid bearing material is a Newtonian fluid. The drag force on the head in shearing liquid, assuming the fluid to be Newtonian, would be: EQU F=Auv/y.
As noted in the '368 patent, the force and power necessary for a non-Newtonian plastic or pseudo-plastic liquid are over an order of magnitude less than required with a Newtonian liquid.
However, head drag is a problem unique to recording apparatus utilizing either a Newtonian or non-Newtonian liquid bearing. Drag is not a problem with air bearing heads because air imparts a drag of only 0.1 g or less on an air bearing head. However, head drag is a significant factor in the movement of the disk under a head with a liquid bearing between them. In particular, achieving a practical recording device utilizing a liquid bearing involves providing a interactive recording head wherein the drag is below a predefined minimum level in relation to the rotational speed of the disk. However, storage technology designers would like the power dissipated due to head drag in liquids to be even lower. Ideally, it is desireable to achieve a head design which would provide reduced drag when used in either a Newtonian or non-Newtonian liquid bearing.
One convenient measurement of the performance of a particular head used in a magnetic storage device is the time between the 50% amplitude points of a pulse signal applied to the head during write and read back during read (generally referred to as PW50). PW50 is an especially accurate reflection of the electrical performance of the head in terms of evaluating heads.
European Patent Application No. 0,367510, published Sep. 5, 1990, teaches a hard disk drive assembly using a low viscosity liquid lubricant on the disk surface to support the slider. Also shown therein is a slider configuration utilizing three triangular pads. One pad is positioned at the leading edge of the slider, while two pads are positioned adjacent the trailing edge of the slider. Each pad is positioned so that an included angle faces the leading edge of he slider, and one side is parallel to the "axis" of the slider. This "inclined" side, forming an angle of about 10.degree. with the included angle, faces the outer edge of the slider with respect to the hub of the disk. It is noteworthy that the bottom surface of each of the pads is at a height of about 25-50 microns above the surface of the slider, and there is no teaching that these pads or any part of any of their surfaces be ramped, or angled with respect to the disk during operation. In addition, the drive assembly specifically requires that the bearing lubricant be maintained at a level of approximately one micron on the disk surface and should not exceed 5 microns (39.4.mu." to 196.5.mu.").